Lignin-preserving bleaching of cellulose pulp

ABSTRACT

A BLEACHING PROCESS FOR CELLULOSE PULPS AND WHICH ENABLES THE LIGNIN IN THE PULP TO BE PRESERVED COMPRISES BLEACHING OF THE PULP IN THE PRESENCE OF AN ORGANIC PEROXIDE, WHICH IS PRODUCED BY CONTACTING A PEROXIDE FORMING ORGANIC COMPOUND WITH AIR OR A GAS CONTAINING FREE OXYGEN.

United States Patent 3,694,309 LIGNIN-PRESERVING BLEACHING 0F CELLULOSEPULP Josef Franz Gierer, Lidingo, and Carl Torbjorn Norin, Bromma,Sweden, assignors to Svenska Traforskningsinstitutet, Stockholm, SwedenNo Drawing. Filed Nov. 10, 1969, Ser. No. 875,564 Claims priority,application Sweden, Nov. 22, 1968, 15,966/68 Int. Cl. D21c US. Cl.162-65 3 Claims ABSTRACT OF THE DISCLOSURE A bleaching process forcellulose pulps and which enables the lignin in the pulp to bepreservedcomprises bleaching of the pulp in the presence of an organicperoxide, which is produced by contacting a peroxide forming organiccompound with air or a gas containing free oxygen.

The present invention relates to a process for bleaching cellulose pulpwhile preserving the lignin therein.

There are known to the art a number of processes intended for bleachingcellulose pulps without adverse effect on the lignin therein. Reductionbleaching with dithionite and oxidizing bleaching with hydrogen peroxideor sodium peroxide are those methods which have obtained the greatestpractical significance.

The reduction bleaching method with dithionite or other reductionagents, such as sulphite or boron hydride, is gentle and imparts to thepulp good properties of strength. The bleaching effect, however, is notlong lasting. Pulps which have been bleached by the reduction methodquickly turn yellow when coming into contact with air, particularly whensubjected to light irradiation or high temperatures.

Oxidative bleaching with hydrogen peroxide or sodium peroxide produces acellulose pulp with a high degree of brightness which is relativelystable, although at the cost of a certain deterioration in themechanical properties of the pulps. The main disadvantages associatedwith peroxide bleaching, however, lie in the instability of thebleaching agent and its high price. In addition to oxidizing, andthereby removing chromophore systems in cellulose pulps, hydrogenperoxide undergoes a spontaneous decomposition which is difficult tocalculate and to control. This decomposition can be reduced to a certainextent, although never entirely eliminated, by the addition ofstabilizers, such as sodium silicate, magnesium sulphate, or complexbuilders, such as for example ethylene diamine-tetraacetic acid (EDTA).The stabilizers and complex builders are not recovered and hence createextra costs. Even when using these additives it is necessary to charge alarge excess of peroxide to the system if the cellulose pulp is toobtain a high degree of brightness.

These disadvantages explain why peroxide bleaching processes havehitherto been mainly applied to cellulose pulps which respond well tobleaching per unit of weight of peroxide used i.e. in single stagebleaching of relatively bright starting pulps (semi-chemical pulps andmechanical wood pulps), in two stage bleaching in combination with areducing bleaching stage (primarily dithionite bleaching) and inmulti-stage bleaching as a terminating bleaching stage.

In accordance with the invention, cellulose pulps can be favourablybleached while preserving the lignin therein by performing the bleachingprocess in the presence of an organic peroxide. The organic peroxide issuitably produced, in accordance with the invention, by using air orsome other oxygen bearing gas and a peroxide forming 3,694,309 PatentedSept. 26, 1972 ice organic compound under correctly adjusted conditionswith regard to pH, temperature, pressure, pulp consistency, reactionrate and, optionally, irradiation.

The process of the invention is suitable as a single-stage bleachingprocess, but may also be used to advantage in processes embodying two ormore bleaching stages. Thus, it is not restricted to types of pulp whichhave a relatively high degree of initial brightness, such as for examplemechanical, semi-chemical and pre-bleached pulps. Particularly whenusing air and easily recovering organic solvents for producing theactive bleaching organic peroxide compounds it is possible in accordancewith the invention to use the process, to advantage, for the partial orcomplete bleaching also of cellulose pulps which present a low initialbrightness, e.g. unbleached sulphate pulp.

Processes for bleaching cellulose pulps with oxygen gas or air are well'known to the art. According to these processes, the cellulose pulp,whilst suspended in caustic soda solution, is treated with air or oxygengas at high temperatures and under pressure. During this treatment themajor portion of the lignin is decomposed and separated and thebleaching process does not preserve the lignin. Furthermore, thistreatment causes undesirable degradation of the cellulose and hence aninferior pulp yield and impaired strength properties of the pulp.

The aim of the process of the invention is, however, to combine theeconomic advantages and effectiveness of, respectively, the air and theoxygen gas bleaching processes with the relative leniency of theperoxide bleaching process. This is achieved by incorporating the oxygeninto organic compounds, to form organic peroxide substances which, in alignin-preserving manner, bleach the cellulose pulps to a high andlasting degree of brightness. The process is lenient in respect of thevalues of the pH and temperature applied, which results therein that theyield losses caused by oxidative and/or hydrolytic decomposition aresmall and that the properties of strength of the resulting, bleachedpulps are well comparable with the corresponding properties of pulpsbleached to the same degree of brightness with hydrogen peroxide in aconventional manner.

' In accordance with the invention, the preparation of the organicperoxidic substances of suitable organic compounds by reaction with airor oxygen bearing gas and reacting the formed peroxidic components withthe chromophore groups of the pulps can take place either in twoseparate processes or simultaneously. In the first case both thepreparation of the bleaching organic peroxide compounds and the reactingthereof can be carried out under optimal conditions in respect of theaforementioned variables. The advantage with this mode of proce-. dureis primarily to be found therein that the pulp need not be subjected tohigh temperatures (and possibly high oxygen gas pressure) for longperiods of time, whereby its strength properties are only slightlyimpaired. The best results have, therefore, been obtained when the stepof preparing the peroxide and the step of reacting it with the cellulosepulp are carried out progressively under reaction conditions which arethe most favourable one for each of these two stages.

It should also be understood, however, that the second mode ofprocedure, i.e. simultaneous preparation and reaction of the peroxidiccompounds with the cellulose pulps, can present certain advantages. Inthis connection, the peroxidic compounds are formed and reacted eitherby introducing air or some other oxygen bearing gas continuously into asuspension of the cellulose pulp in a peroxide forming organic compoundunder the conditions given below, or by treating the pulp suspensionwith air or oxygen bearing gas under pressure in a closed .system. Sincethis process does at the same time involve an eifective extraction ofthe cellulose pulps, i.e. a removal of lipophilic and hydrophilic lowmolecular constituents, e.g. resins, steroids, fats, monoandoligosaccharides, there are obtained in addition to the bleachingeffect, the same advantages as those when treated with solely the saidcompound (solvent). This combined bleaching and extraction effect isparticularly advantageous in connection with lignin-preserving bleachingof mechanical, thermomechanical and semi-mechanical pulps which presenta relatively high content of the extractive substances. Furthermore, theuse of watersoluble, peroxide forming organic solvents has the advantagethat the pulps are effectively dewatered during the bleaching operation.

In accordance with the invention, peroxide forming organic compounds arereacted with oxygen gas or oxygen bearing gas, preferably air, and theresulting peroxides are reacted with cellulose pulps. The following willdiscuss more closely the most suitable conditions for preparing theperoxides and their reaction with cellulose pulps.

Many peroxide forming organic compounds can be used for bleachingcellulose pulps while preserving the lignin therein. The best results,however, are obtained with organic compounds which with free oxygen giveWater-soluble peroxidic compounds, particularly such compounds whichreadily form organic hydroperoxides and which together with water swellthe cellulose pulps and thereby render them more readily accessible toattacks by the organic peroxides. Dioxan, a commercially availablesolvent which is produced on a large scale, has the aforementionedproperties. It forms together with air or free oxygen bearing gas awater-soluble hydroperoxide (M.P. 53-56 C.) and swells cellulose pulps.It can readily be recovered by distillation and be reused for the sametype of lignin preserving bleaching process. The majority of thebleaching tests in accordance with the invention have therefore beencarried out with dioxan as the peroxide forming organic compound. Itshould be mentioned, however, that also other peroxide forming organiccompounds, such as other cyclic ethers such as tetrahydrofuran,tetrahydropyran, non-cyclic ethers and alcohols, e.g. isopropanol,Z-butanol, give satisfactory bleaching results.

Preparation of the bleaching organic peroxides by a separate processprior to the cellulose bleaching process is effected either by passingair or free oxygen containing gas through the peroxide forming compoundat high temperature, preferably at the boiling point of the compound, orby heating the compound with air or oxygen bearing gas at pressures inexcess of atmospheric in a closed system, at temperatures between theboiling point of the compound and 50 C. thereabove, generally between 80and 150 C. Since the rate at which the peroxide is formed depends uponthe amount of oxygen gas which is dissolved in the peroxide formingorganic compound, and since this quantity is proportional to the partialpressure of oxygen over the compound, it is preferred for economicreasons to prepare the compound at pressures in excess of atmospheric.Generally an air pressure of between 3-50 atmospheres above atmosphericpressure is used, corresponding to an oxygen gas pressure of between06-10 atmospheres above atmospheric pressure, preferably -30 and 4-6atmospheres above atmospheric pressure respectively. The use of oxygengas instead of air shortens the reaction rate for obtaining a specificperoxide content to approximately between one third and one fifth.

During the formation of peroxide, the pH falls because of the formationof acid decomposition products from the peroxidic compounds. Since thedecomposition of the organic peroxides, active during the bleachingprocess, is catalysed by protons, the formation of the peroxides iscompensated for after a certain length of time by the decomposition,i.e. the peroxide content passes through a maximum. For the purpose ofavoiding, or at least delaying, decomposition of the peroxides they can,in accordance with the invention, be prepared in the presence ofstabilizers, such as sodium or magnesium salts, for instance, sodiumsilicate, sodium carbonate, magnesium sulphate etc. Such salts have beenfound essential when preparing peroxides in large quantities while usinglong reaction times.

If an open system is employed, the rate at which the peroxides areformed can be further increased by irradiating the system with UV-lightin the presence of a sensibilizer, e.g. benzophenone.

In principal, all types of cellulose pulps can be bleached by applyingthe method of the present invention. The

majority of tests have heretofore been made with unbleached sulphatepulps which present a low initial brightness and with unbleached 'orpre-bleached neutral pulphite pulps and mechanical pulps havingrelatively high degree of initial brightness. The pulp consistency canbe varied within wide limits e.g. from 1 to 25%. In practice aconsistency of approximately 8% (8 g. of pulp/ ml.) has been foundadvantageous. Under otherwise equal conditions the bleaching result is,primarily, determined by the relationship of organic peroxide to pulp.

The reaction between the organic peroxides and the cellulose pulps issuitably effected so that the brightness is increased by the largestpossible extent within the shortest possible time. The optimum bleachingconditions vary of course with different types of pulp and differentorganic peroxidic compounds. Normally, temperatures between 40-100 C.and reaction times between 0.5-4 hours are applied. In the case oflonger reaction times an addition of stabilizers, e.g. sodium silicate,provides a favourable effect on the bleaching result.

It is evident from the aforegoing that the optimal conditions for,firstly, preparing the organic peroxides and, secondly, their reactionwith cellulose pulps are not identical. If the preparation of theperoxides and their reaction with the pulps is to take placesimultaneously. i.e. in one and the same process stage, the selection ofthe optimal bleaching conditions must therefore be based on compromises,primarily with respect to temperature, pressure, reaction time and pH.These process variables are suitably selected so that the highestpossible increase in brightness is obtained with the least possibledecomposition of the cellulose and of the lignin. Of course, the optimalbleaching conditions in this method of procedure are also dependent onthe type of pulp to be bleached and on which peroxide forming organiccompound and oxygen bearing gas is to be used.

Normally the cellulose pulps are suspended in the peroxide formingorganic compound and bleaching is effected by introducing air or oxygenbearing gas through the suspension, heated to the boiling point of thecompound, or by heating the suspension with air or oxygen bearing gas atpressures of atmospheres above atmospheric pressure (between 3 and 50 inrespect of air, and 06-10 in respect of oxygen gas) to the boiling pointof the compound or up to 50 C. above said boiling point in a closedsystem (autoclave tube). The reaction time is primarily determined bythe degree of brightness to be obtained and the extent to whichdeterioration in the mechanical properties of the pulp can be tolerated.In this method of procedure reaction times of between 5 and 50 hours arenormally chosen.

The invention will now be described with reference to a number ofexamples which illustrate the influence of different process variableson the formation of the organic peroxides, on their reaction with thedifferent cellulose pulps and on the optical and mechanical propertiesof the bleached pulps.

EXAMPLE 1 Oxygen gas or air was introduced into 250 ml. of the peroxideforming organic compound during a reflux di- 6 EXAMPLE 3 Mechanical pulpfrom spruce (50 g.) having an initial brightness of 58.8% G.E. and anintrinsic viscosity of 153 cp. was suspended in 625 ml. water containingsodium silicate (2.5 g.) and peroxide (4 g.), prepared from TABLEl.-PEROXIDE FORMATION IN DIFFERENT ORGANIC COM- POUNDS WITH AIR OROXYGEN GAS AT BOILING POINT Peroxide With Ml consumed forming or without.1 N organic Temper- Reaction benzo- NazSzOa compound Air or ature, timephenone solution (250 ml.) oxygen (hours) (50 mg.) per ml.

Isopropanol Air 82 68 1.60 D0 Air 82 27 1.60 2-butanol 100 24 25.50Tetrahydrofuran. 65 48 10.00 Tetrahydropyran. 81 60 15.00 D0 81 10360.00 Dioxan... 100 24 12.50 Do--. 100 18.10 D0.-- 100 24 4.50 Do.-. 10048 10.00 Do... 100 24 8.10 D0 100 30 12.30

EXAMPLE 2 dioxan, was added to the suspenslon. Bleaching was In ordertoshow the effect of different stabilizers on the formation of peroxidethe method of procedure in accordance with Example 1 was repeated withdioxan effected under the conditions disclosed in Table 3, which alsoshows the values of brightness, viscosity and the most importantmechanical properties of some of the bleached mechanical pulps.

TABLE 3 [Bleaching of mechanical pulp (50 g.) (58.8% G.E., 153 cp.) withperoxide from dioxan (4 g.) in the presence of sodium silicate (2.5 g)]Reaction Brightness Intrinsic Beating Breaking Rupture time (percentviscosity degree length strength Tearing Double Temperature, 0. (hours)G.E.) (cp.) S.R.) (m.) (kp./cm. factor ioldings Unbleachedmechanicalpulp (250 ml.) and oxygen gas at 100 C., with the addition of differentinorganic salts.

-TABLE 2.-FORM.ATION OF PEROXIDE IN DIOXAN WITH OXYGEN GAS AT 100 0. INTHE PRESENCE OF INOR- oaNro sat/rs Reaction ml. consumed Nags O4 (150).-

Tests at 40 C. and a reaction time of 1 hour gave a bleached pulp havinga brightness of 66.1% G.E.

EXAMPLE 4 A neutral sulphite pulp from spruce (50' g.) having an initialbrightness of 48.6% G.E. and an intrinsic viscosity of 781 cp. wassuspended in 625 ml. of water, optionally containing sodium silicate(2.5 g.), and was treated in the manner described in Example .3. Table 4discloses the bleaching conditions and the brightness; intrinsicviscosity and mechanical properties of the bleached pulps.

TABLE 4 [Bleaching 01 neutral sulphlte pulp 50 g.) (48.6% G.E., 781 .53peroxide trom dioxan (4 g.) with and without sodliim silicate WithBright- Reaction or wi ness Intrinsic Beating Breaking Rupture time out(percen viscosity degree length strength Tearing Double Temperature, 0.(hours) NazSiO; G.E.) (cp.) S.R.) (m.) (kp./cm.'-) factor toldingsUnbleached neutral sulphite pulp 781 43 9, 437 7. 1 64 1, 574

Test with the bleaching of a corresponding pulp with washed withdistilled water, dried in air and examined in hydrogen peroxide at thetemperature of 60 C. and a respect of brightness, intrinsic viscosityand mechanical reaction time of 4 hours showed that bleaching toacproperties. The results are disclosed in Table 6.

TABLE 6 [Bleaching of different types of pulp (40 g.) with air in dioxan(21.) during refluxed digestion for 48 hours.']

With

With or without without benzo- Brightness Intrinsic Beating BreakingRupture $11 810; g)henone (percent viscosity); degree length strengthTearing Double g. cp.

Type of pulp (40 g.) (2 4 g.) G.E.) 8.3..) (m.) (kp./cm. factor ioldings1 Corresponding values for the starting pulps are found in Tables 3-5.

ceptable brightness can only be achieved in the presence EXAMPLE 7 of astabilizer.

EXAMPLE Oxygen gas was forced into an autoclave tube made of stainlesssteel and containing a suspension of pulp (40 Unbleached sulphate pulpfrom pine (50 g.) having an g.) in dioxan (21.), and optionally sodiumsilicate (2 g.) initial brightness of 23% G.E. and an intrinsicviscosity as a stabilizer, to a pressure of 5 atmospheres above of 1128cp. was suspended in 625 ml. of water, optionally atmospheric pressure.The sample was rotated for 6, 12 containing sodium silicate (2.5 g.),and was treated in 25 and 23 hours, respectively, in a polyglycol bathheated the manner described in Example 3. Table 5 discloses to 100 C.The obtained pulp was washed with distilled the bleaching conditions andthe values of the brightness, water, dried in air and examined inrespect of brightness. intrinsic viscosity and mechanical properties ofthe Table 7 discloses the measuring values and the bleaching bleachedsulphate pulps. For comparison purposes, the conditions.

TABLE 7 [Bleaching of different types of pulp g. of each) in diolzsafi((32 11) with oxygen gas (5 atmospheres above atmospheric pressure) Withorwithout Reaction Bright- Intrinsic Beating Breaking Rupture Na-1S1O;time ness(perviscosity degree length strength Tearing Double Type ofpulp (40 g.) (2 g.) (hours) cent G.E.) (cp.) 8.11.) (m.) (kpJcm factorioldings Mechanical pulp. 23 Neutral sulphite pul 23 1 Correspondingvalues for the starting pulps are given in Tables 3-5. 3 Treated withair instead of oxygen.

table also discloses corresponding values for the un- What is claimedis: bleached sulphate pulp. 1. A process for bleaching cellulose pulpswhile pre- TABLE 5 [Bleaching of sulphate pulp g.) (23.2% G.E., 1,128cp.) with peroxide from dioxan (4 g.) with and without sodium silicate(2.5 g.)]

With Bright- Reaction or withness Intrinsic Beating Breaking Rupturetime out (percent viscosity degree length strength Tearing DoubleTemperature, 0. (hours) NazSiOa G.E.) (op. 8.3..) (m.) (kpJcmJ) factorfoldings Unbleached sulphate pulp Tests with the bleaching of acorresponding pulp with serving the lignin therein comprising adding tounhydrogen peroxide at a temperature of C. and a rebleached cellulosepulp an organic compound selected action time of 4 hours resultedtherein that bleaching to from the group consisting of alcohols andethers, conan acceptable brightness can only be achieved in the tactingthe cellulose pulp and organic compound with presence of a stabilizer.free oxygen under a pressure of from about 0.6 to about EXAMPLE 6 10atmospheres above atmospheric pressure at a temperature of from about 80C. to about 150 C. during Dltferent types of pulp (40 g. of each type)were a reaction time from about 5 to about 50 hours whereby suspended indioxan (2 optionally containing sodium an organic peroxide is formed andthe cellulose pulp is sihcate (2 g.) as a stabilizer and/or benzophenone(0.4 bleached thereby in one stage.

g.) as a sensibllrzer, and the suspension was refluxed 2. A process asclaimed in claim 1 wherein the organic digested on the water bath duringconstant percolation of compound is an ether selected from the groupconsisting air for 48 hours. The pulp was subjected to filtration, ofdioxan, tetrahydrofuran and tetrahydropyran.

9 3. A process as claimed in claim 1 wherein the 01'- 2,042,705 6/ 1936Dreyfus 162-72 ganic compound is an alcohol selected from the group3,514,278 5/ 1970 Brink, Jr 16278 X consisting of isopropanol andZ-butanol. OTHER REFERENCES R f s Cit d 5 Chemistry of OrganicCompounds, Noller, 1951, p. UNITED STATES PATENTS 1,767,543 6/ 1930McKee et a1. 8-111 HOWARD R. CAINE, Primary 'Examiner 3,458,394 7/1969Yiannos et a1. 162-78 X Us Cl XR 2,939,813 6/ 1960 Wayman et a1.162----78 2,022,664 12/1935 Groombridge et a1. 162-72 X 10 8-111; 16272,77, 78

